The efficiency of an axial-throughflow gas turbine is influenced, inter alia, by leakage streams of the compressed gas that occur between rotating and nonrotating components of the turbine. The gap occurring between the tips of the moving blades and the casing walls surrounding the moving blades plays an appreciable part in this. Efforts are therefore aimed at keeping the gaps as small as possible. In the event of deviation from the design point, a brushing of the moved components against the static components can easily occur. For this reason, use is often made of brushing- and/or abrasion-tolerant structural elements, such as, for example, honeycomb seals, honeycombs or else porous ceramic or metallic structures or felts, which serve as counterrunning surfaces of the sealing tips of the moving blades and are partially cut into by these during a running-in phase. Use of such brushing-tolerant sealing elements reduces serious machine damage in the event of minor brushing events, since the brushing is absorbed by the soft structure of the counterrunning surface, without the blades being damaged.
Both the tips of the moving blades or guide vanes and the honeycomb seals used are exposed to very high temperatures when the gas turbine is operating in the hot-gas mode.
It is therefore known, for example from U.S. Pat. No. 3,365,172, to act upon the sealing tips of the moving blades through honeycomb seals with cooling air. For this purpose, the carrier for the honeycomb seals is pierced through with small cooling air bores that are supplied with cooling air via a peripheral annular chamber.
JP 61149506 shows a similar embodiment, in which the honeycomb seals are carried by a layer of porous metal that is contiguous to a supply chamber for cooling air. In this embodiment, too, the cooling air is delivered to the blade tips through the honeycomb seals.
The routing of cooling air through porous sealing elements is likewise known from U.S. Pat. No. 6,171,052. In this case, the porous sealing elements are transpiration-cooled by the cooling air when the latter flows through them. U.S. Pat. No. 4,013,376 discloses a configuration in which the counterrunning surface of the blades is designed to be both impact-cooled and transpiration-cooled. U.S. Pat. No. 3,728,039 likewise discloses transpiration-cooled porous rings as counterrunning surfaces of blades. In this case, the feed of cooling air to the ring is segmented. The ring itself is produced in one piece.
One problem with a multiplicity of configurations is that, when, due to brushing, damage to the gas-permeable elements occurs or even a region is torn out completely, the coolant pressure collapses, and overheating and finally the failure of the entire sealing arrangement occur. Likewise, when, in a region, the porosity is blocked due to deformation induced by brushing or else due to dirt, the coolant flows around this region of the sealing element. The cooling of the latter is no longer ensured, and local overheating occurs. Due to the overheating, the region affected may burn up. The cooling air then flows out through the large hole which has thus arisen, and the previously unaffected regions are no longer cooled. The component as a whole consequently fails over the entire circumference.
A further challenge that arises is to use the available cooling air as efficiently as possible, since, by virtue of a saving of cooling air, considerable power output and efficiency potentials can be exploited.